Data from: Multicellularity drives the evolution of sexual traits

From the male peacock's tail plumage to the floral displays of
flowering plants, traits related to sexual reproduction are often complex
and exaggerated. Why has sexual reproduction become so complicated? Why
have such exaggerated sexual traits evolved? Early work posited a
connection between multicellularity and sexual traits such as anisogamy
(i.e., the evolution of small sperm and large eggs). Anisogamy then drives
the evolution of other forms of sexual dimorphism. Yet, the relationship
between multicellularity and the evolution of sexual traits has not been
empirically tested. Given their extensive variation in both multicellular
complexity and sexual systems, the volvocine green algae offer a tractable
system for understanding the interrelationship of multicellular complexity
and sex. Here we show that species with greater multicellular complexity
have a significantly larger number of derived sexual traits, including
anisogamy, internal fertilization, and secondary sexual dimorphism. Our
results demonstrate that anisogamy repeatedly evolved from isogamous
multicellular ancestors and that anisogamous species are larger and
produce larger zygotes than isogamous species. In the volvocine algae, the
evolution of multicellularity likely drives the evolution of anisogamy,
and anisogamy subsequently drives secondary sexual dimorphism.
Multicellularity may set the stage for the overall diversity of sexual
complexity throughout the Tree of Life.